There is "no question" of Olympics security being compromised, the home secretary has said, after it emerged 3,500 extra servicemen would be needed.

Theresa May said it was discovered only on Wednesday - 16 days before the Games begin - that contractor G4S did not have enough trained security staff.

The servicemen are in addition to 7,500 already agreed for venue security.

Labour MP Keith Vaz said: "G4S has let the country down and we have literally had to send in the troops."

G4S was contracted by the London 2012 Organising Committee to supply 10,400 staff out of the 23,700 security staff needed for the Games.

It said it had 4,000 people already working across 100 venues.

It also said there were a further 9,000 people going through the final stages of extensive training, vetting and accreditation.

It is understood that it was training more people than their contract required to make up for any shortfall in staff taking up their posts.

"We have encountered some delays in progressing applicants through the final stages but we are working extremely hard to process these as swiftly as possible," the company said.

The remaining security staff were to be made up of 7,500 from the military, and about 5,800 students and volunteers.

Mrs May addressed the Commons in response to an urgent question from Mr Vaz about the safety during the Olympics.

'Most important' task

Mrs May responded: "I can confirm to the House that there remains no specific security threat to the Games and the threat level remains unchanged.

"And let me reiterate that there is no question of Olympic security being compromised."

Mrs May said Britain had the "finest military personnel in the world".

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Media captionKeith Vaz quizzed Theresa May on why "we literally had to send in the troops"

"They stand ready to do their duty whatever the nation may ask. Our troops are highly skilled and highly trained and this task is the most important facing our nation today," she said.

Shadow home secretary Yvette Cooper said the scale of the problem should have been recognised sooner.

She welcomed the decision to bring in extra servicemen, but added: "This really looks like another huge Home Office shambles."

In a statement, Downing Street warned the company should face consequences for its failure to fulfil its contract - for which it is being paid £284m.

The London 2012 Organising Committee (Locog) said it was not commenting on contractual arrangements with G4S.

'Adverse impact'

Defence Secretary Phillip Hammond said he had authorised the deployment of the 3,500 military personnel, bringing the total number of military personnel - from three services and including reservists - contributing to Games security to 17,000.

London 2012 - One extraordinary year

Personnel had already begun to be deployed to venues "to support the rolling search and lock-down process between now and the start of the Olympics, alongside the police, the commercial security provider, G4S, and volunteers," he said in his written ministerial statement to Parliament.

Eleven thousand of them will be involved in security at around 30 sporting venues, carrying out checks at venue gates and doing bag checks, and some 70 non-competition venues, including car parks and hotels.

Some 5,000 personnel will be in specialist roles, such as bomb disposal squads, special forces and the London missile sites, with a further 1,000 involved in logistical support.

Mr Hammond said the Chiefs of Staff supported the deployment, adding it would not have an adverse impact on other operations.

"We will ensure that all those taking part receive their full leave entitlement, even if it has to be rescheduled, that no-one is out of pocket due to cancelled personal arrangements and that all deployed personnel are appropriately supported," he said.

Mrs May gave further details, saying 10,000 Olympic and Paralympic tickets had been donated to the armed services, there was access for 2,000 spectators in the Mall for road events, and troops had the right to buy 2,000 Olympic Park tickets.

The heavy military presence at the Olympic Park will likely divide opinion

Chief Inspector of Borders John Vine said some staff were not dealing with arriving passengers efficiently or effectively enough and that forged documents could be going undetected.

Downing Street said G4S's inability to provide enough security for the Olympics was "unfortunate".

The prime minister's spokesman said a joint military-civilian security operation was always envisaged at the Olympics and "we are happy to change that balance."

The spokesman said the government would meet the cost of the redeployment of the servicemen but refused to say whether Locog would force G4S to recompense the taxpayer.

Mrs May said the government remained confident the venue security costs would remain within the £553m budget.

One woman, Catherine Roberts, said she was now unemployed after being turned down for a position with G4S in Cardiff, despite being given two weeks of training in London in June.

She said although she was told she had passed all the necessary tests and training, her employment was terminated - first because she was told she had failed a course and then because of incorrect documentation.

"The money they have wasted on sending me to London and through courses that cost £200 to then lay me off is ridiculous. I am absolutely gutted as I have been left jobless with no way of paying for my rent."

The company was also one of four contractors bidding to take over some services for Surrey Police, but the region's police authority voted on Thursday to suspend the plan - and is now thinking of scrapping it entirely.

Members of Surrey Police Authority were unhappy about G4S's failure to recruit enough guards for the Olympics.